/* SPDX-License-Identifier: GPL-2.0+ */
/*
 * RCU expedited grace periods
 *
 * Copyright IBM Corporation, 2016
 *
 * Authors: Paul E. McKenney <paulmck@linux.ibm.com>
 */

#include <linux/lockdep.h>

static void rcu_exp_handler(void *unused);
static int rcu_print_task_exp_stall(struct rcu_node *rnp);

/*
 * Record the start of an expedited grace period.
 */
static void rcu_exp_gp_seq_start(void)
{
        rcu_seq_start(&rcu_state.expedited_sequence);
}

/*
 * Return then value that expedited-grace-period counter will have
 * at the end of the current grace period.
 */
static __maybe_unused unsigned long rcu_exp_gp_seq_endval(void)
{
        return rcu_seq_endval(&rcu_state.expedited_sequence);
}

/*
 * Record the end of an expedited grace period.
 */
static void rcu_exp_gp_seq_end(void)
{
        rcu_seq_end(&rcu_state.expedited_sequence);
        smp_mb(); /* Ensure that consecutive grace periods serialize. */
}

/*
 * Take a snapshot of the expedited-grace-period counter.
 */
static unsigned long rcu_exp_gp_seq_snap(void)
{
        unsigned long s;

        smp_mb(); /* Caller's modifications seen first by other CPUs. */
        s = rcu_seq_snap(&rcu_state.expedited_sequence);
        trace_rcu_exp_grace_period(rcu_state.name, s, TPS("snap"));
        return s;
}

/*
 * Given a counter snapshot from rcu_exp_gp_seq_snap(), return true
 * if a full expedited grace period has elapsed since that snapshot
 * was taken.
 */
static bool rcu_exp_gp_seq_done(unsigned long s)
{
        return rcu_seq_done(&rcu_state.expedited_sequence, s);
}

/*
 * Reset the ->expmaskinit values in the rcu_node tree to reflect any
 * recent CPU-online activity.  Note that these masks are not cleared
 * when CPUs go offline, so they reflect the union of all CPUs that have
 * ever been online.  This means that this function normally takes its
 * no-work-to-do fastpath.
 */
static void sync_exp_reset_tree_hotplug(void)
{
        bool done;
        unsigned long flags;
        unsigned long mask;
        unsigned long oldmask;
        int ncpus = smp_load_acquire(&rcu_state.ncpus); /* Order vs. locking. */
        struct rcu_node *rnp;
        struct rcu_node *rnp_up;

        /* If no new CPUs onlined since last time, nothing to do. */
        if (likely(ncpus == rcu_state.ncpus_snap))
                return;
        rcu_state.ncpus_snap = ncpus;

        /*
         * Each pass through the following loop propagates newly onlined
         * CPUs for the current rcu_node structure up the rcu_node tree.
         */
        rcu_for_each_leaf_node(rnp) {
                raw_spin_lock_irqsave_rcu_node(rnp, flags);
                if (rnp->expmaskinit == rnp->expmaskinitnext) {
                        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                        continue;  /* No new CPUs, nothing to do. */
                }

                /* Update this node's mask, track old value for propagation. */
                oldmask = rnp->expmaskinit;
                rnp->expmaskinit = rnp->expmaskinitnext;
                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

                /* If was already nonzero, nothing to propagate. */
                if (oldmask)
                        continue;

                /* Propagate the new CPU up the tree. */
                mask = rnp->grpmask;
                rnp_up = rnp->parent;
                done = false;
                while (rnp_up) {
                        raw_spin_lock_irqsave_rcu_node(rnp_up, flags);
                        if (rnp_up->expmaskinit)
                                done = true;
                        rnp_up->expmaskinit |= mask;
                        raw_spin_unlock_irqrestore_rcu_node(rnp_up, flags);
                        if (done)
                                break;
                        mask = rnp_up->grpmask;
                        rnp_up = rnp_up->parent;
                }
        }
}

/*
 * Reset the ->expmask values in the rcu_node tree in preparation for
 * a new expedited grace period.
 */
static void __maybe_unused sync_exp_reset_tree(void)
{
        unsigned long flags;
        struct rcu_node *rnp;

        sync_exp_reset_tree_hotplug();
        rcu_for_each_node_breadth_first(rnp) {
                raw_spin_lock_irqsave_rcu_node(rnp, flags);
                WARN_ON_ONCE(rnp->expmask);
                rnp->expmask = rnp->expmaskinit;
                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
        }
}

/*
 * Return non-zero if there is no RCU expedited grace period in progress
 * for the specified rcu_node structure, in other words, if all CPUs and
 * tasks covered by the specified rcu_node structure have done their bit
 * for the current expedited grace period.  Works only for preemptible
 * RCU -- other RCU implementation use other means.
 *
 * Caller must hold the specificed rcu_node structure's ->lock
 */
static bool sync_rcu_preempt_exp_done(struct rcu_node *rnp)
{
        raw_lockdep_assert_held_rcu_node(rnp);

        return rnp->exp_tasks == NULL &&
               READ_ONCE(rnp->expmask) == 0;
}

/*
 * Like sync_rcu_preempt_exp_done(), but this function assumes the caller
 * doesn't hold the rcu_node's ->lock, and will acquire and release the lock
 * itself
 */
static bool sync_rcu_preempt_exp_done_unlocked(struct rcu_node *rnp)
{
        unsigned long flags;
        bool ret;

        raw_spin_lock_irqsave_rcu_node(rnp, flags);
        ret = sync_rcu_preempt_exp_done(rnp);
        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

        return ret;
}


/*
 * Report the exit from RCU read-side critical section for the last task
 * that queued itself during or before the current expedited preemptible-RCU
 * grace period.  This event is reported either to the rcu_node structure on
 * which the task was queued or to one of that rcu_node structure's ancestors,
 * recursively up the tree.  (Calm down, calm down, we do the recursion
 * iteratively!)
 *
 * Caller must hold the specified rcu_node structure's ->lock.
 */
static void __rcu_report_exp_rnp(struct rcu_node *rnp,
                                 bool wake, unsigned long flags)
        __releases(rnp->lock)
{
        unsigned long mask;

        for (;;) {
                if (!sync_rcu_preempt_exp_done(rnp)) {
                        if (!rnp->expmask)
                                rcu_initiate_boost(rnp, flags);
                        else
                                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                        break;
                }
                if (rnp->parent == NULL) {
                        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                        if (wake) {
                                smp_mb(); /* EGP done before wake_up(). */
                                swake_up_one(&rcu_state.expedited_wq);
                        }
                        break;
                }
                mask = rnp->grpmask;
                raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled */
                rnp = rnp->parent;
                raw_spin_lock_rcu_node(rnp); /* irqs already disabled */
                WARN_ON_ONCE(!(rnp->expmask & mask));
                rnp->expmask &= ~mask;
        }
}

/*
 * Report expedited quiescent state for specified node.  This is a
 * lock-acquisition wrapper function for __rcu_report_exp_rnp().
 */
static void __maybe_unused rcu_report_exp_rnp(struct rcu_node *rnp, bool wake)
{
        unsigned long flags;

        raw_spin_lock_irqsave_rcu_node(rnp, flags);
        __rcu_report_exp_rnp(rnp, wake, flags);
}

/*
 * Report expedited quiescent state for multiple CPUs, all covered by the
 * specified leaf rcu_node structure.
 */
static void rcu_report_exp_cpu_mult(struct rcu_node *rnp,
                                    unsigned long mask, bool wake)
{
        unsigned long flags;

        raw_spin_lock_irqsave_rcu_node(rnp, flags);
        if (!(rnp->expmask & mask)) {
                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                return;
        }
        rnp->expmask &= ~mask;
        __rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */
}

/*
 * Report expedited quiescent state for specified rcu_data (CPU).
 */
static void rcu_report_exp_rdp(struct rcu_data *rdp)
{
        WRITE_ONCE(rdp->deferred_qs, false);
        rcu_report_exp_cpu_mult(rdp->mynode, rdp->grpmask, true);
}

/* Common code for work-done checking. */
static bool sync_exp_work_done(unsigned long s)
{
        if (rcu_exp_gp_seq_done(s)) {
                trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done"));
                /* Ensure test happens before caller kfree(). */
                smp_mb__before_atomic(); /* ^^^ */
                return true;
        }
        return false;
}

/*
 * Funnel-lock acquisition for expedited grace periods.  Returns true
 * if some other task completed an expedited grace period that this task
 * can piggy-back on, and with no mutex held.  Otherwise, returns false
 * with the mutex held, indicating that the caller must actually do the
 * expedited grace period.
 */
static bool exp_funnel_lock(unsigned long s)
{
        struct rcu_data *rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
        struct rcu_node *rnp = rdp->mynode;
        struct rcu_node *rnp_root = rcu_get_root();

        /* Low-contention fastpath. */
        if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
            (rnp == rnp_root ||
             ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
            mutex_trylock(&rcu_state.exp_mutex))
                goto fastpath;

        /*
         * Each pass through the following loop works its way up
         * the rcu_node tree, returning if others have done the work or
         * otherwise falls through to acquire ->exp_mutex.  The mapping
         * from CPU to rcu_node structure can be inexact, as it is just
         * promoting locality and is not strictly needed for correctness.
         */
        for (; rnp != NULL; rnp = rnp->parent) {
                if (sync_exp_work_done(s))
                        return true;

                /* Work not done, either wait here or go up. */
                spin_lock(&rnp->exp_lock);
                if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) {

                        /* Someone else doing GP, so wait for them. */
                        spin_unlock(&rnp->exp_lock);
                        trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
                                                  rnp->grplo, rnp->grphi,
                                                  TPS("wait"));
                        wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
                                   sync_exp_work_done(s));
                        return true;
                }
                rnp->exp_seq_rq = s; /* Followers can wait on us. */
                spin_unlock(&rnp->exp_lock);
                trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
                                          rnp->grplo, rnp->grphi, TPS("nxtlvl"));
        }
        mutex_lock(&rcu_state.exp_mutex);
fastpath:
        if (sync_exp_work_done(s)) {
                mutex_unlock(&rcu_state.exp_mutex);
                return true;
        }
        rcu_exp_gp_seq_start();
        trace_rcu_exp_grace_period(rcu_state.name, s, TPS("start"));
        return false;
}

/*
 * Select the CPUs within the specified rcu_node that the upcoming
 * expedited grace period needs to wait for.
 */
static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
{
        int cpu;
        unsigned long flags;
        unsigned long mask_ofl_test;
        unsigned long mask_ofl_ipi;
        int ret;
        struct rcu_exp_work *rewp =
                container_of(wp, struct rcu_exp_work, rew_work);
        struct rcu_node *rnp = container_of(rewp, struct rcu_node, rew);

        raw_spin_lock_irqsave_rcu_node(rnp, flags);

        /* Each pass checks a CPU for identity, offline, and idle. */
        mask_ofl_test = 0;
        for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
                unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
                struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
                int snap;

                if (raw_smp_processor_id() == cpu ||
                    !(rnp->qsmaskinitnext & mask)) {
                        mask_ofl_test |= mask;
                } else {
                        snap = rcu_dynticks_snap(rdp);
                        if (rcu_dynticks_in_eqs(snap))
                                mask_ofl_test |= mask;
                        else
                                rdp->exp_dynticks_snap = snap;
                }
        }
        mask_ofl_ipi = rnp->expmask & ~mask_ofl_test;

        /*
         * Need to wait for any blocked tasks as well.  Note that
         * additional blocking tasks will also block the expedited GP
         * until such time as the ->expmask bits are cleared.
         */
        if (rcu_preempt_has_tasks(rnp))
                rnp->exp_tasks = rnp->blkd_tasks.next;
        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

        /* IPI the remaining CPUs for expedited quiescent state. */
        for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
                unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
                struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);

                if (!(mask_ofl_ipi & mask))
                        continue;
retry_ipi:
                if (rcu_dynticks_in_eqs_since(rdp, rdp->exp_dynticks_snap)) {
                        mask_ofl_test |= mask;
                        continue;
                }
                ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
                if (!ret) {
                        mask_ofl_ipi &= ~mask;
                        continue;
                }
                /* Failed, raced with CPU hotplug operation. */
                raw_spin_lock_irqsave_rcu_node(rnp, flags);
                if ((rnp->qsmaskinitnext & mask) &&
                    (rnp->expmask & mask)) {
                        /* Online, so delay for a bit and try again. */
                        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                        trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("selectofl"));
                        schedule_timeout_uninterruptible(1);
                        goto retry_ipi;
                }
                /* CPU really is offline, so we can ignore it. */
                if (!(rnp->expmask & mask))
                        mask_ofl_ipi &= ~mask;
                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
        }
        /* Report quiescent states for those that went offline. */
        mask_ofl_test |= mask_ofl_ipi;
        if (mask_ofl_test)
                rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false);
}

/*
 * Select the nodes that the upcoming expedited grace period needs
 * to wait for.
 */
static void sync_rcu_exp_select_cpus(void)
{
        int cpu;
        struct rcu_node *rnp;

        trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("reset"));
        sync_exp_reset_tree();
        trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("select"));

        /* Schedule work for each leaf rcu_node structure. */
        rcu_for_each_leaf_node(rnp) {
                rnp->exp_need_flush = false;
                if (!READ_ONCE(rnp->expmask))
                        continue; /* Avoid early boot non-existent wq. */
                if (!READ_ONCE(rcu_par_gp_wq) ||
                    rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
                    rcu_is_last_leaf_node(rnp)) {
                        /* No workqueues yet or last leaf, do direct call. */
                        sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work);
                        continue;
                }
                INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
                cpu = find_next_bit(&rnp->ffmask, BITS_PER_LONG, -1);
                /* If all offline, queue the work on an unbound CPU. */
                if (unlikely(cpu > rnp->grphi - rnp->grplo))
                        cpu = WORK_CPU_UNBOUND;
                else
                        cpu += rnp->grplo;
                queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
                rnp->exp_need_flush = true;
        }

        /* Wait for workqueue jobs (if any) to complete. */
        rcu_for_each_leaf_node(rnp)
                if (rnp->exp_need_flush)
                        flush_work(&rnp->rew.rew_work);
}

static void synchronize_sched_expedited_wait(void)
{
        int cpu;
        unsigned long jiffies_stall;
        unsigned long jiffies_start;
        unsigned long mask;
        int ndetected;
        struct rcu_node *rnp;
        struct rcu_node *rnp_root = rcu_get_root();
        int ret;

        trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
        jiffies_stall = rcu_jiffies_till_stall_check();
        jiffies_start = jiffies;

        for (;;) {
                ret = swait_event_timeout_exclusive(
                                rcu_state.expedited_wq,
                                sync_rcu_preempt_exp_done_unlocked(rnp_root),
                                jiffies_stall);
                if (ret > 0 || sync_rcu_preempt_exp_done_unlocked(rnp_root))
                        return;
                WARN_ON(ret < 0);  /* workqueues should not be signaled. */
                if (rcu_cpu_stall_suppress)
                        continue;
                panic_on_rcu_stall();
                pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
                       rcu_state.name);
                ndetected = 0;
                rcu_for_each_leaf_node(rnp) {
                        ndetected += rcu_print_task_exp_stall(rnp);
                        for_each_leaf_node_possible_cpu(rnp, cpu) {
                                struct rcu_data *rdp;

                                mask = leaf_node_cpu_bit(rnp, cpu);
                                if (!(rnp->expmask & mask))
                                        continue;
                                ndetected++;
                                rdp = per_cpu_ptr(&rcu_data, cpu);
                                pr_cont(" %d-%c%c%c", cpu,
                                        "O."[!!cpu_online(cpu)],
                                        "o."[!!(rdp->grpmask & rnp->expmaskinit)],
                                        "N."[!!(rdp->grpmask & rnp->expmaskinitnext)]);
                        }
                }
                pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
                        jiffies - jiffies_start, rcu_state.expedited_sequence,
                        rnp_root->expmask, ".T"[!!rnp_root->exp_tasks]);
                if (ndetected) {
                        pr_err("blocking rcu_node structures:");
                        rcu_for_each_node_breadth_first(rnp) {
                                if (rnp == rnp_root)
                                        continue; /* printed unconditionally */
                                if (sync_rcu_preempt_exp_done_unlocked(rnp))
                                        continue;
                                pr_cont(" l=%u:%d-%d:%#lx/%c",
                                        rnp->level, rnp->grplo, rnp->grphi,
                                        rnp->expmask,
                                        ".T"[!!rnp->exp_tasks]);
                        }
                        pr_cont("\n");
                }
                rcu_for_each_leaf_node(rnp) {
                        for_each_leaf_node_possible_cpu(rnp, cpu) {
                                mask = leaf_node_cpu_bit(rnp, cpu);
                                if (!(rnp->expmask & mask))
                                        continue;
                                dump_cpu_task(cpu);
                        }
                }
                jiffies_stall = 3 * rcu_jiffies_till_stall_check() + 3;
        }
}

/*
 * Wait for the current expedited grace period to complete, and then
 * wake up everyone who piggybacked on the just-completed expedited
 * grace period.  Also update all the ->exp_seq_rq counters as needed
 * in order to avoid counter-wrap problems.
 */
static void rcu_exp_wait_wake(unsigned long s)
{
        struct rcu_node *rnp;

        synchronize_sched_expedited_wait();
        rcu_exp_gp_seq_end();
        trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end"));

        /*
         * Switch over to wakeup mode, allowing the next GP, but -only- the
         * next GP, to proceed.
         */
        mutex_lock(&rcu_state.exp_wake_mutex);

        rcu_for_each_node_breadth_first(rnp) {
                if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
                        spin_lock(&rnp->exp_lock);
                        /* Recheck, avoid hang in case someone just arrived. */
                        if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
                                rnp->exp_seq_rq = s;
                        spin_unlock(&rnp->exp_lock);
                }
                smp_mb(); /* All above changes before wakeup. */
                wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rcu_state.expedited_sequence) & 0x3]);
        }
        trace_rcu_exp_grace_period(rcu_state.name, s, TPS("endwake"));
        mutex_unlock(&rcu_state.exp_wake_mutex);
}

/*
 * Common code to drive an expedited grace period forward, used by
 * workqueues and mid-boot-time tasks.
 */
static void rcu_exp_sel_wait_wake(unsigned long s)
{
        /* Initialize the rcu_node tree in preparation for the wait. */
        sync_rcu_exp_select_cpus();

        /* Wait and clean up, including waking everyone. */
        rcu_exp_wait_wake(s);
}

/*
 * Work-queue handler to drive an expedited grace period forward.
 */
static void wait_rcu_exp_gp(struct work_struct *wp)
{
        struct rcu_exp_work *rewp;

        rewp = container_of(wp, struct rcu_exp_work, rew_work);
        rcu_exp_sel_wait_wake(rewp->rew_s);
}

#ifdef CONFIG_PREEMPT_RCU

/*
 * Remote handler for smp_call_function_single().  If there is an
 * RCU read-side critical section in effect, request that the
 * next rcu_read_unlock() record the quiescent state up the
 * ->expmask fields in the rcu_node tree.  Otherwise, immediately
 * report the quiescent state.
 */
static void rcu_exp_handler(void *unused)
{
        unsigned long flags;
        struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
        struct rcu_node *rnp = rdp->mynode;
        struct task_struct *t = current;

        /*
         * First, the common case of not being in an RCU read-side
         * critical section.  If also enabled or idle, immediately
         * report the quiescent state, otherwise defer.
         */
        if (!t->rcu_read_lock_nesting) {
                if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
                    rcu_dynticks_curr_cpu_in_eqs()) {
                        rcu_report_exp_rdp(rdp);
                } else {
                        rdp->deferred_qs = true;
                        set_tsk_need_resched(t);
                        set_preempt_need_resched();
                }
                return;
        }

        /*
         * Second, the less-common case of being in an RCU read-side
         * critical section.  In this case we can count on a future
         * rcu_read_unlock().  However, this rcu_read_unlock() might
         * execute on some other CPU, but in that case there will be
         * a future context switch.  Either way, if the expedited
         * grace period is still waiting on this CPU, set ->deferred_qs
         * so that the eventual quiescent state will be reported.
         * Note that there is a large group of race conditions that
         * can have caused this quiescent state to already have been
         * reported, so we really do need to check ->expmask.
         */
        if (t->rcu_read_lock_nesting > 0) {
                raw_spin_lock_irqsave_rcu_node(rnp, flags);
                if (rnp->expmask & rdp->grpmask) {
                        rdp->deferred_qs = true;
                        t->rcu_read_unlock_special.b.exp_hint = true;
                }
                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                return;
        }

        /*
         * The final and least likely case is where the interrupted
         * code was just about to or just finished exiting the RCU-preempt
         * read-side critical section, and no, we can't tell which.
         * So either way, set ->deferred_qs to flag later code that
         * a quiescent state is required.
         *
         * If the CPU is fully enabled (or if some buggy RCU-preempt
         * read-side critical section is being used from idle), just
         * invoke rcu_preempt_deferred_qs() to immediately report the
         * quiescent state.  We cannot use rcu_read_unlock_special()
         * because we are in an interrupt handler, which will cause that
         * function to take an early exit without doing anything.
         *
         * Otherwise, force a context switch after the CPU enables everything.
         */
        rdp->deferred_qs = true;
        if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
            WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs())) {
                rcu_preempt_deferred_qs(t);
        } else {
                set_tsk_need_resched(t);
                set_preempt_need_resched();
        }
}

/* PREEMPT=y, so no PREEMPT=n expedited grace period to clean up after. */
static void sync_sched_exp_online_cleanup(int cpu)
{
}

/*
 * Scan the current list of tasks blocked within RCU read-side critical
 * sections, printing out the tid of each that is blocking the current
 * expedited grace period.
 */
static int rcu_print_task_exp_stall(struct rcu_node *rnp)
{
        struct task_struct *t;
        int ndetected = 0;

        if (!rnp->exp_tasks)
                return 0;
        t = list_entry(rnp->exp_tasks->prev,
                       struct task_struct, rcu_node_entry);
        list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
                pr_cont(" P%d", t->pid);
                ndetected++;
        }
        return ndetected;
}

#else /* #ifdef CONFIG_PREEMPT_RCU */

/* Invoked on each online non-idle CPU for expedited quiescent state. */
static void rcu_exp_handler(void *unused)
{
        struct rcu_data *rdp;
        struct rcu_node *rnp;

        rdp = this_cpu_ptr(&rcu_data);
        rnp = rdp->mynode;
        if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
            __this_cpu_read(rcu_data.cpu_no_qs.b.exp))
                return;
        if (rcu_is_cpu_rrupt_from_idle()) {
                rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
                return;
        }
        __this_cpu_write(rcu_data.cpu_no_qs.b.exp, true);
        /* Store .exp before .rcu_urgent_qs. */
        smp_store_release(this_cpu_ptr(&rcu_data.rcu_urgent_qs), true);
        set_tsk_need_resched(current);
        set_preempt_need_resched();
}

/* Send IPI for expedited cleanup if needed at end of CPU-hotplug operation. */
static void sync_sched_exp_online_cleanup(int cpu)
{
        struct rcu_data *rdp;
        int ret;
        struct rcu_node *rnp;

        rdp = per_cpu_ptr(&rcu_data, cpu);
        rnp = rdp->mynode;
        if (!(READ_ONCE(rnp->expmask) & rdp->grpmask))
                return;
        ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
        WARN_ON_ONCE(ret);
}

/*
 * Because preemptible RCU does not exist, we never have to check for
 * tasks blocked within RCU read-side critical sections that are
 * blocking the current expedited grace period.
 */
static int rcu_print_task_exp_stall(struct rcu_node *rnp)
{
        return 0;
}

#endif /* #else #ifdef CONFIG_PREEMPT_RCU */

/**
 * synchronize_rcu_expedited - Brute-force RCU grace period
 *
 * Wait for an RCU grace period, but expedite it.  The basic idea is to
 * IPI all non-idle non-nohz online CPUs.  The IPI handler checks whether
 * the CPU is in an RCU critical section, and if so, it sets a flag that
 * causes the outermost rcu_read_unlock() to report the quiescent state
 * for RCU-preempt or asks the scheduler for help for RCU-sched.  On the
 * other hand, if the CPU is not in an RCU read-side critical section,
 * the IPI handler reports the quiescent state immediately.
 *
 * Although this is a greate improvement over previous expedited
 * implementations, it is still unfriendly to real-time workloads, so is
 * thus not recommended for any sort of common-case code.  In fact, if
 * you are using synchronize_rcu_expedited() in a loop, please restructure
 * your code to batch your updates, and then Use a single synchronize_rcu()
 * instead.
 *
 * This has the same semantics as (but is more brutal than) synchronize_rcu().
 */
void synchronize_rcu_expedited(void)
{
        struct rcu_data *rdp;
        struct rcu_exp_work rew;
        struct rcu_node *rnp;
        unsigned long s;

        RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
                         lock_is_held(&rcu_lock_map) ||
                         lock_is_held(&rcu_sched_lock_map),
                         "Illegal synchronize_rcu_expedited() in RCU read-side critical section");

        /* Is the state is such that the call is a grace period? */
        if (rcu_blocking_is_gp())
                return;

        /* If expedited grace periods are prohibited, fall back to normal. */
        if (rcu_gp_is_normal()) {
                wait_rcu_gp(call_rcu);
                return;
        }

        /* Take a snapshot of the sequence number.  */
        s = rcu_exp_gp_seq_snap();
        if (exp_funnel_lock(s))
                return;  /* Someone else did our work for us. */

        /* Ensure that load happens before action based on it. */
        if (unlikely(rcu_scheduler_active == RCU_SCHEDULER_INIT)) {
                /* Direct call during scheduler init and early_initcalls(). */
                rcu_exp_sel_wait_wake(s);
        } else {
                /* Marshall arguments & schedule the expedited grace period. */
                rew.rew_s = s;
                INIT_WORK_ONSTACK(&rew.rew_work, wait_rcu_exp_gp);
                queue_work(rcu_gp_wq, &rew.rew_work);
        }

        /* Wait for expedited grace period to complete. */
        rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
        rnp = rcu_get_root();
        wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
                   sync_exp_work_done(s));
        smp_mb(); /* Workqueue actions happen before return. */

        /* Let the next expedited grace period start. */
        mutex_unlock(&rcu_state.exp_mutex);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);